As integrated circuit feature size continues to shrink, more functional blocks are integrated in a single chip. As such, circuit testing becomes more challenging. To minimize the number of defects undetected during manufacture testing, various test pattern sets for a wide range of fault models must be applied, thereby increasing test pattern counts significantly. The resultant high test data volume leads to high test costs. Reducing test data volume, therefore, has been an important area for circuit testing technology development.
On-chip test compression has become a standard design-for-test approach in industry today. The original idea of test data compression exploits the fact that the number of specified bits in test cubes is typically no more than 1% of total number of scan cells in the design. To achieve test compression, these specified bits are encoded as a LFSR (linear feedback shift register) seed. During a test, the encoded seed is decompressed by an on-chip LFSR and shifted into scan chains.
The encoding capacity can be improved by combining a linear finite-state machine (e.g., a ring generator) with a combinational linear decompressor (e.g., an XOR network). Compressed test stimuli, injected at both current and previous cycles, are utilized to encode the test stimulus needed at the current shift cycle. During a test, the compressed test stimuli are injected continuously while the decompressed test stimuli are shifted into scan chains. This dynamic reseeding approach enables a greater encoding capability than a static reseeding approach.
The encoding efficiency can be further improved through vector clustering. In this scheme, incompatible test cubes are merged to form a parent test cube and incremental test patterns along with a control pattern. During test data loading, the parent test pattern is combined with the incremental test patterns according to the control pattern to reconstruct the incompatible test cubes.
Still further test data volume reduction can be achieved and will be discussed below based on additional properties of test cubes.